Multi-compartment overfire air and N-agent injection method and system for nitrogen oxide reduction in flue gas

ABSTRACT

A method of decreasing a concentration of nitrogen oxides in a combustion gas flowing through a vessel including: generating a flue gas in a combustion zone of the vessel, the flue gas containing nitrogen oxides and carbon monoxide; providing overfire air into a burnout zone of the vessel from a first injector of overfire air to oxidize at least some of the carbon monoxide in the flue gas; injecting a selective reducing agent concurrent with overfire air at a level in the burnout zone downstream of the first injector of overfire air and downstream of the oxidization of the carbon monoxide, and reacting the selective reducing agent with the flue gas to reduce the nitrogen oxides.

CROSS RELATED APPLICATION

This application is a divisional of application Ser. No. 10/454,597,filed Jun. 5, 2003, which application is incorporated in its entirety byreference herein.

BACKGROUND OF THE INVENTION

This invention relates generally to reducing emission of nitrogen oxidesfrom combustion systems, such as boilers, furnaces and incinerators.

A group of air pollutants produced by combustion in boilers and furnacesinclude oxides of nitrogen, mainly NO and NO₂. Nitrogen oxides (NO_(X))are the subject of growing concern because of their toxicity and theirrole as precursors in acid rain and photochemical smog processes.Reduction of nitrogen oxides has been the focus of many technologydevelopment efforts.

In modern boilers and furnaces and other such combustion vessels,emissions of nitrogen oxides (NO_(X)) have been greatly reduced by theuse of overfire air (“OFA”) technology. In this technology, most of thecombustion air goes into the combustion chamber together with the fuel,but addition of a portion of the combustion air is delayed to yieldoxygen lean conditions initially and then to facilitate combustion of COand any residual fuel.

Selective Non-Catalytic Reduction (“SNCR”) technologies reduce NO_(X) incombustion gas by injecting a nitrogenous reducing agent (“N-agent”),such as ammonia or urea, into the gas. The N-agent is injected at hightemperature and under conditions such that a non-catalytic reactionselectively reduces NO_(X) to molecular nitrogen. Reduction of NO_(X) isselective because the molecular nitrogen in the combustion gas is notreduced, while the NO_(X) is reduced by the N-agent.

The N-agent is typically released into flue gas that is within anoptimum temperature range or window, such as between 1700 degrees to2200 degrees Fahrenheit (930 to 1200 degree Celsius). The flue gas oftenhas moderate to high carbon monoxide (CO) concentrations (0.2-1.0percent). In some SNCR applications, the CO in flue gas chemicallycompetes with the active species in the N-agent needed for NO_(X)reduction. This competition reduces the effectiveness of the SNCRprocess and NO_(X) reduction, and/or moves the optimum temperaturewindow to lower temperatures.

Earlier SNCR techniques circumvented the CO problem by spraying largeN-agent droplets into overfire air injected into the flue gas. As theOFA and flue gas steams mix, CO is oxidized and water in the dropletsevaporates as the droplets are carried to cooler regions of the boiler.This process delays the release of the N-agent until the gas temperaturehas reached the optimal temperature window.

Large droplet N-agent systems have difficulties that can reduce theireffectiveness such as: long droplet residence times in the flue gas, atortuous flow path with obstructions for the droplets, and a narrowN-agent release temperature window. If the droplets are too small, theyrelease the N-agent upstream of the optimal temperature window where theflue gas is still too hot and render the N-agent ineffective. Underthese conditions, the N-agent can generate (rather than reduce) NO_(X).On the other hand, if the droplets are too large, a portion of theN-agent is released after the combustion gas has cooled below theoptimal temperature window causing high ammonia concentrations (ammoniaslip) in the flue gas outlet stream. Finally, there is a need for betterSNCR techniques to address the problems raised by high CO concentrationsin the flue gas near the droplet injection location.

SUMMARY OF THE INVENTION

In one embodiment, the invention is a method of decreasing theconcentration of nitrogen oxides in a combustion gas flowing downstreamthrough a vessel, comprising: generating a flue gas in a combustion zoneof the vessel, the flue gas partly composed of nitrogen oxides andcarbon monoxide; injecting overfire air into a burnout zone of thevessel from a first source of overfire air to oxidize at least some ofthe carbon monoxide in the flue gas; spraying a selective reducing agentconcurrently with overfire air into a burnout zone downstream of thefirst source of overfire air and downstream of the oxidization of thecarbon monoxide; and reacting the selective reducing agent with the fluegas to reduce the nitrogen oxides.

In a second embodiment, the invention is a combustion vessel having acombustion zone; a burnout zone downstream of the combustion zone; anoverfire air compartment adjacent the burnout zone, wherein the overfireair compartment has an upstream air injector and a downstream airinjector, and at least one agent injector for injecting a selectivereducing agent into the burnout zone wherein the agent injector isplaced in the air injector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a coal-fired combustion vessel.

FIG. 2 is a schematic diagram of a multi-compartment overfire chamberfor the vessel shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a schematic representation of a combustion system 10 such as thatused in a coal-fired boiler or furnace. The combustion system 10includes a combustion vessel 11 having a combustion zone 12, a burnoutzone 14 and an optional reburning zone 16. The combustion zone 12includes one or more main burners 18 mounted on at least one of thewalls 20 of combustion vessel 11. The walls form a vertical chamber forthe combustion zone 12, reburning zone 16, burnout zone 14 and othercomponents in the flue gas stream of the system 10.

The main burners are supplied with a main fuel, such as coal, directlyor through a fuel manifold 22 and with air directly or through an airbox 24. The air box may be mounted on the outside of the walls 20opposite to the combustion zone 12 inside the vessel. The air box is amanifold that distributes air to each of the burners.

Combustion of the fuel injected by the main burners 18 and air from theair box 24 occurs in the combustion zone 12 of the vessel. The flue gas26 produced by the combustion flows in a downstream direction that isupward from the combustion zone 12 to the burnout zone 14 in the vessel11. The main burners supply the heat energy input into the vessel.Additional heat may be released into the vessel 11 at the reburning zone16 where a reburn fuel, such as natural gas, is combusted. The reburnfuel enters the vessel 11 through a reburn fuel injector 28.

Downstream of reburning zone 16 is the burnout zone 14 where overfireair enters the vessel 11 through an overfire air injector 30. Downstreamof the burnout zone in the vessel 11, the flue gas 26 optionally passesthrough a series of heat exchangers 32. Solid particles remaining in theflue gas may be removed by a particulate control device 33, such as anelectrostatic precipitator (“ESP”) or baghouse.

A selective reducing agent (N-agent) is sprayed into the burnout zone 14with the overfire air. An N-agent injector (nozzle and lance) is placedin the overfire air chamber 30 and injects the selective reducing agentinto the burnout zone 14 along with overfire air. As used herein, theterms “selective reducing agent” and “N-agent” are used interchangeablyto refer to any of a variety of chemical species capable of selectivelyreducing NO_(X) in the presence of oxygen in a combustion system. Ingeneral, suitable selective reducing agents include urea, ammonia,cyanuric acid, hydrazine, thanolamine, biuret, triuret, ammelide,ammonium salts of organic acids, ammonium salts of inorganic acids, andthe like. Specific examples of ammonium salt reducing agents include,ammonium sulfate, ammonium bisulfate, ammonium bisulfite, ammoniumformate, ammonium carbonate, ammonium bicarbonate, ammonium nitrate, andthe like. Mixtures of these selective reducing agents can also be used.The selective reducing agent is provided in a solution, preferably anaqueous solution, or in the form of a powder. One selective reducingagent is urea in aqueous solution.

As shown in FIG. 2, the overfire air input chamber 30 includes aplurality of OFA injectors 34, 36. These injectors are in regions of thechamber 30 from which overfire air flows through the wall 20 and intothe burnout zone 14 of the vessel 11. The overfire chamber 30 isattached to the wall 20 of the vessel.

The OFA injectors of the chamber 30 are arranged vertically one over theother on the wall 20 of the vessel. A lower OFA injector 34 (upstreaminjector in flue gas) of the chamber 30 is a conduit that provides air,e.g., at a high flow rate, into the burnout zone 14. An upper OFAinjector 36 (downstream injector in flue gas) of the chamber 30 alsoprovides air to the burnout zone. The overfire air supplied by thedownstream injector may be at a reduced flow rate than the air flowingthrough the upstream injector. Each of the OFA injectors may have wallsthat define an air conduit through which air flows to the wall 20 of thevessel, through penetrations in the wall and into the burnout zone 14 ofthe vessel.

A separator plate 46 in the chamber 30 may provide a wall separating theupper and lower OFA injectors. However, a separator plate may not beneeded if the OFA injectors are not contained in one air input chamber30, but are separated from one another with some of the vessel wall 20between the OFA injectors. There may be more than two OFA air injectors,but the injector furthest downstream will generally include the N-agentinjector. For example, two or more upstream OFA injectors may supply airto the burnout zone 14 and a final downstream OFA injector with anN-agent injector may supply both overfire air and the N-agent to theburnout zone 14.

The air from the upstream injector reduces the CO concentration in theburnout zone 14, before the N-agent is released. Air from the downstreaminjector 36 flows into the burnout zone 14 with the droplets containingthe N-agent. The air mass flow through the upstream OFA injector(s) maybe substantially greater than the mass flow through the downstream OFAinjector. The flow rates of air through each of the injectors may becontrolled to regulate the amount of overfire air flowing into thevessel. Adjustable dampers 44 in each of the injectors 34, 36 may beused to regulate the amount of air flowing through each injector.Similarly, fans may be positioned in the overfire chamber 30 upstream ofthe injector and used to move air into the overfire chamber at controlflow rates.

N-agent nozzles 38 spray the N-agent into the burnout zone. Each N-agentnozzle 38 is placed at the end of a lance 48 that extends through thedownstream overfire air injector in the overfire chamber 30. There maybe a plurality e.g., three or four, of the agent injectors and lancesarranged in the wall 20 and through the downstream OFA injector 36.N-agent is introduced into the burnout zone 14 through the N-agentnozzle 38 concurrently with the air flowing through the downstream OFAinjector 36. The N-agent flows downstream as the OFA mixes with thecombustion gas 26. Once released, the N-agent chemically reacts withcombustion gas to reduce the NO_(X) emissions.

Flue gas 26, with moderate to high CO concentrations, flows upward fromthe combustion zone into the burnout zone 14 where they initially mixwith the overfire air from the lower compartment 34 and subsequently mixwith the N-agent and overfire air from the upper compartment 36. Thecarbon monoxide (CO) in the flue gas flowing from the combustion andreburning zones 12, 16 is oxidized in the burnout zone 14 by the airflowing from the lower compartment 34 of the overfire chamber 30. Oxygen(O₂) in the air reacts with the CO to form carbon dioxide. The oxidationof the CO occurs in the burnout zone 14 upstream (below) the level wherethe N-agent is injected.

By injecting air into the vessel through the upstream injector 34 thatis below the N-agent injector 38, a substantial portion of the carbonmonoxide in the flue gas 26 is oxidized before the gas comes intocontact with the N-agent. The oxidization of the CO upstream of theN-agent injection location may allow the N-agent to be sprayed into theflue gas with smaller droplets sizes reducing droplet residence times inthe flue gas.

Airflow rates in the upper and lower injectors 34, 36 are adjusted toshield the N-agent from the flue gas until a sufficient amount of theflue gas CO is oxidized by the air from the lower compartment 34. Thisusually requires that more air flow through the upstream injector 34than the downstream injector 36. For example, the air mass flowingthrough the upstream injector 34 may be four to ten times the air massflowing through the downstream injector 36. The low CO concentration inthe flue gas that contacts the N-agent improves N-agent effectiveness byreducing the competition between CO and NO_(X) for active speciescritical to SNCR NO_(X) reduction chemistry.

The N-agent injector 38 may be a nozzle at the end of a lance 48 thatextends through the downstream injector 36. An input end of the lance,opposite to the nozzle 38, is coupled to a source of the N-agent. Theremay be multiple agent nozzles and lances for N-agent injectors arrangedin the upper chamber and along the wall 20 of the vessel 11. The N-agentinjector may be positioned at a level of the vessel 11 corresponding toa desired temperature of the flue gas in the burnout zone. For example,the agent injector 38 may be at a level where the temperature of theflue gas is in a range of 1,700 to 2,500 degrees Fahrenheit. The N-agentnozzle 38 may inject small droplets or gas of N-agent into the burnoutzone. The small droplets release the N-agent to the combustion gasquicker than do larger droplets.

Pilot-scale field tests have demonstrated the negative effect that CO incombustion gas has on SNCR NO_(X) reduction chemistry. The presence of2000 parts-per-million (ppm) of CO in the combustion gas has been shownto effectively eliminate the NO_(X) reduction achieved with N-Agentinjection. For example, pilot-scale field tests conducted on a 300 kW(kiloWatt) cylindrical coal-fired furnace indicate that the N-agentreduces NO_(X) in combustion gas by 6 to 25 percent when CO is notpresent in the flue gas. However, the NO_(X) reduction due to theN-agent becomes negligible when CO at 2000 ppm is present in the fluegas. Accordingly, reduction of CO in the combustion gas is a factor thatimproves NO_(X) reduction when using SNCR technology.

Computational Fluid Dynamic (CFD) computer simulations of a typicalboiler furnace demonstrated that to reduce NO_(X) by injecting anN-agent in overfire air, the temperature of the combustion gas enteringthe burnout zone should be in the temperature range from 1700 degrees to2500 degrees Fahrenheit. The N-agent should be injected as smalldroplets into the gas and a split flow overfire air chamber 30 shouldprovide substantially greater air mass flow through a lower compartment34 than through the upper compartment 36. The split in the air mass flowbetween the upstream and downstream compartments in the overfire chambermay be as great as 10 to 1, where this ratio means that ten times asmuch air mass flows through the lower compartment as flows in the uppercompartment. The CFD results showed that NO_(X) was reduced by 21percent when the air mass split was 4 to 1, and NO_(X) was reduced by 35percent when the air mass split was 10 to 1. The relative adjustment ofthe air flow rate may be performed by moving dampers 44 in the upper andlower injectors, or adjusting the speed of fans driving air into theupper and lower injectors.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A combustion system comprising: a vertical chamber defining ancombustion zone internal to the chamber; a burner adjacent the verticalchamber and supplying fuel to the combustion zone; an overfire air ductsupplying overfire air to the combustion chamber, wherein the overfireair is supplied downstream of the combustion zone; wherein the overfireair duct includes a lower passage for overfire air and an upper passagefor overfire air, and injecting a reducing agent into the overfire airin the upper passage initial and not injecting the reducing agent intothe overfire air in the lower passage.
 2. A combustion system as inclaim 1 wherein the reducing agent is selected from a group consistingof urea, ammonia, cyanuric acid, hydrazine, thanolamine, biuret,triuret, ammelide, ammonium salts of organic acids, ammonium salts ofinorganic acids, ammonium salt, ammonium sulfate, ammonium bisulfate,ammonium bisulfite, ammonium formate, ammonium carbonate, ammoniumbicarbonate and ammonium nitrate.
 3. A combustion system as in claim 1wherein the overfire air duct is adjacent the vertical chamber and theduct has an exit which opens into an internal flue gas passage of thevertical chamber.
 4. A combustion system as in claim 1 wherein theoverfire air duct is above a reburning zone in the vertical chamber. 5.A combustion system as in claim 1 wherein the lower passage and upperpassage are defined by a separator plate in the overfire air duct.
 6. Acombustion system as in claim 1 wherein the reducing agent is injectedinto the upper overfire air as the upper overfire air enters thecombustion chamber.
 7. A combustion boiler for combusting fuel andgenerating heat, and a reducing agent being added to the combustionboiler so as to minimize discharge of nitrogen oxides from thecombustion boiler, the combustion boiler comprising: a housing definingan internal combustion chamber therein; at least one fuel supply ductconnected to the combustion boiler for supplying fuel to the combustionchamber; and at least one initial overfire air duct supplying an initialoverfire air stream to the combustion chamber to facilitate completecombustion of the fuel supplied to the combustion boiler; wherein the atleast one initial overfire air duct is divided into an initial primaryoverfire air stream and an initial secondary overfire air stream, and areducing agent is directly sprayed only into the initial secondaryoverfire air stream, and not into the initial primary overfire airstream, and the reducing agent mixes with the initial overfire airstream.
 8. The combustion boiler according to claim 7, wherein thehousing comprises a base wall, a sidewall and a top wall with an exitsection formed in the sidewall adjacent the top wall, and an indentationis formed in the sidewall of the housing to form a throat.
 9. Thecombustion boiler according to claim 7, wherein the reducing agent isselected from a group consisting of urea, ammonia, cyanuric acid,hydrazine, thanolamine, biuret, triuret, ammelide, ammonium salts oforganic acids, ammonium salts of inorganic acids, ammonium salt,ammonium sulfate, ammonium bisulfate, ammonium bisulfite, ammoniumformate, ammonium carbonate, ammonium bicarbonate and ammonium nitrate.10. A combustion boiler for combustion of fuel and generating heat, thecombustion boiler comprising: a housing defining an internal combustionchamber therein; at least one fuel supply duct connected to thecombustion boiler for supplying fuel to the combustion chamber; and atleast one initial overfire air duct for supplying an initial overfireair stream to the combustion chamber to facilitate complete combustionof the fuel supplied to the combustion boiler; wherein an overfire airduct is divided into an primary overfire air stream and a secondaryoverfire air stream; a reducing agent is added only to the secondaryoverfire air stream, but not the primary overfire air stream, and thereducing agent added to the initial secondary initial overfire airstream is selected from a group consisting of urea, ammonia, cyanuricacid, hydrazine, thanolamine, biuret, triuret, ammelide, ammonium saltsof organic acids, ammonium salts of inorganic acids, ammonium salt,ammonium sulfate, ammonium bisulfate, ammonium bisulfite, ammoniumformate, ammonium carbonate, ammonium bicarbonate and ammonium nitrate.11. A combustion boiler for combusting fuel and generating heat, and areducing agent being added to the combustion boiler so as to minimizedischarge of nitrogen oxides from the combustion boiler, the combustionboiler comprising: a housing defining an internal combustion chambertherein; at least one fuel supply duct connected to the combustionboiler for supplying fuel to the combustion chamber; and at least oneinitial overfire air duct supplying an initial overfire air stream tothe combustion chamber to facilitate complete combustion of the fuelsupplied to the combustion boiler; wherein the at least one initialoverfire air duct is divided into an initial primary overfire air streamand an initial secondary overfire air stream; a reducing agent isdirectly sprayed only into the initial secondary overfire air stream,but not the initial primary overfire air stream, and allowed to mix withthe initial overfire air stream, and the housing comprises a base wall,a sidewall and a top wall with an exit section formed in the sidewalladjacent the top wall, and an indentation formed in the sidewall of thehousing to form a throat which accelerates combustion byproducts and theoverfire air stream and any residual reducing agent as the combustionbyproducts, the overfire air stream and any residual reducing agent flowfrom a combustion chamber toward a secondary chamber located above theindentation in the combustion boiler.
 12. The combustion boileraccording to claim 11, wherein the reducing agent added to the initialsecondary overfire air stream is selected from a group comprisingammonia, ammonium salts of organic acids, and ammonium salts ofinorganic acids and urea.